Apart from the compressed feel, what are your other qualms with the modeling they present?
Well, for me, the compression is already enough. For my taste, it's just no tasty "sag" or whatever, but digital with a tendency to even produce artefacts (just turn the natural amp model up so it starts to get hairy and play some notes in the higher midrange through a bridge pickup).
Also, while certainly related to the compression issue, there's no great marshall-ish amp for lower gains. X-Crunch is actually a sort of decent amp (something I will use when running the GT without additional sidecars), but once you're in lower gain areas, the compression gets very noticeable for me (and unfortunately it's something I can't "unfeel") - and that's precisely where Marshalls rule, there's still lots of dynamics even if they're quite hairy already.
In addition, the amps don't take external pedals very well. I think this is again at least partially related to the compression happening, I already noticed that in the past with some things that were notoriously un-dynamic, such as the first and second generation PODs (and fwiw, quite ironically, the better dynamics have been the reason for me to choose a GT-5 over a POD 2 and a GT-10 over a POD XT back in the days), but also with some models in the Amplifirebox.
And then there's the cab situation. Not only is it kinda awkward to deal with for a start (especially in case you're using IRs), the internal cabs are pretty strange, too. They really only seem to work well with the internal amps. Been running some of my HX favourites through them and it really sounded pretty bad. The IR functionality is quite weird, too. Not only is the loading process as convoluted as it gets (they're really doing this through the MIDI protocol as it seems - which is completely unbelievable in 2024), IRs also don't seem to sound the same once loaded in the GT (I'll do some more thorough exploration when I find the time).
Add to this that neither the internal cabs nor the IR slots offer any kind of simple EQ, not even a low or high cut. That's pretty, pretty bad.
So, all these combined make up for a pretty much less than shiny experience.
And fwiw, it's of course really just a matter of taste, you can certainly get things done using just the Boss amps, but when I connected the Amplifirebox and later on the HX Stomp and did some comparisons, it was absolutely night and day for me.
And while I will defenitely create some patches for running the GT just on its own, I will defenitely use some additional stuff whenever I can - which is quite saddening as there's some gigs I play here and there, which defenitely don't need much stuff to be carried around. Maybe I'll get a Tonex One for just those particular gigs and use it for the dirts.